| # | Network | Host range | Broadcast | Hosts |
|---|
An IPv4 address is 32 bits, usually written as four octets (0–255 each). CIDR notation like
/24 says how many of those leading bits are the network — fixed for everyone on the
subnet — leaving the rest for individual hosts. A /24 fixes 24 bits and leaves 8, giving
256 addresses (254 usable). Move the slider and watch the block grow or shrink by powers of two.
Take 192.168.1.100/24. The /24 mask
(255.255.255.0) keeps the first three octets, so the network is
192.168.1.0 and the broadcast is 192.168.1.255. Usable hosts run
.1 to .254 — 254 of them.
Split that /24 into /26s and you get four subnets of 62 hosts each:
.0, .64, .128 and .192. That's how a flat network gets divided between, say, four VLANs.
Every block reserves its first address as the network identifier and its last as the broadcast address, so a /24's 256 addresses give 254 usable hosts. The exceptions are /31 (a two-address point-to-point link) and /32 (a single host).
It's the inverse of the subnet mask — 1s where the mask has 0s. Access-control lists and some routing tools (notably Cisco) use it instead of the subnet mask to describe the same range.
Three blocks are reserved for private use: 10.0.0.0/8, 172.16.0.0/12, and
192.168.0.0/16. Addresses inside them aren't routable on the public internet — they're for
your own networks.
Not yet — this is IPv4 only. IPv6 uses the same CIDR idea but with 128-bit addresses, so host counts become astronomically large and the "usable = total − 2" rule no longer applies.